UK. Patent 2,046,556B describes a communication system involving portable cordless telephone units which co-operate with base units which are terminations on the public switched telephone networks (PSTN) to provide public call-office type facilities for the portable cordless telephone units.
In such systems the portable cordless telephone units are self powered and to conserve power they have a cycle of 1.4 seconds, during which time they mostly try to "sleep", using as little power as possible. At least once every 1.4 s, portable units will wake-up and scan the channels to see if any base unit is trying to contact them. If they detect a base unit contacting them, they then stay awake and start interpreting the signalling being sent to them.
The base unit is responsible for polling the portable units it wants to ring when a new incoming call is detected by the base unit. In between each poll to a portable unit, the base unit sends a small signalling packet, which contains information to polled portable units, telling them whether or not to ring. Typically, a base will tell the portable unit to ring when it is detecting incoming ringing, and tell them to keep quiet while the quiet period between ringing bursts occurs.
Because portable units will be sleeping most of the time, and wake up in their own time (they are not synchronised), it is not predictable when a handset detects its own poll and starts ringing. The effect of this is that a population of handsets will appear to start ringing at random intervals, when they wake-up.
The random ringing of the portable units from a base unit is considered as undesirable particularly in multiple portable unit group ringing circumstances.